Critical evaluation of BNBC seismic response spectrum for Chattogram using one-dimensional nonlinear site response analysis

Abstract Accurate estimation of ground motion at the surface is fundamental to seismic hazard assessment and resilient infrastructure design. The Bangladesh National Building Code (BNBC) 2020 provides generalized seismic response spectra by scaling bedrock motions with amplification factors based on soil classification. However, the applicability and conservativeness of these spectra in urban centers like Chattogram—situated in Seismic Zone II—remain insufficiently validated through site-specific analyses. This study conducts a rigorous one-dimensional (1D) site response analysis using both nonlinear (NL) and equivalent linear (EL) methods across 18 out of 22 geotechnical boreholes which represent soil type SD representative of Chattogram’s subsurface conditions. Ten ground motion records, selected and scaled from the Pacific Earthquake Engineering Research (PEER) Center database, were input at the engineering bedrock level (~ 125 m depth), as inferred from a prior microtremor survey indicating shear wave velocities of ~ 800 m/s. DEEPSOIL software was employed to model soil behavior under seismic loading. Results reveal that for soil type SD, the average surface PGA derived from nonlinear analysis is 0.2680 g, closely aligning with BNBC 2020’s MCE-level estimates (0.28 g). In contrast, the equivalent linear method significantly overestimates PGA at 0.4405 g, suggesting potential misrepresentation of soil nonlinearity in simplified approaches. Comparison of 5% damped response spectra further indicates that the BNBC 2020 design spectrum for Soil Type SD tends to be non-conservative at medium-to-long periods, while the equivalent linear approach remains overly conservative across much of the period range. These discrepancies highlight the need for code revisions to incorporate nonlinear effects explicitly, particularly for soft-to-stiff soil profiles prevalent in the region. This study provides critical insights into the limitations of current design spectra in BNBC 2020 and underscores the necessity for site-specific response modeling to enhance seismic resilience in Chattogram and similar urban settings in Bangladesh.